Today I wondered what the first organism to evolve vision would have been. I assume that it would have been kind of primitive and basic, but of course extremely innovative and eventually useful to a wide variety of beings.

It also makes sense that the evolution of vision would have accompanied the evolution of advanced brain functions in almost every case. So, do biologists have any idea as to what the first species to use vision would have been?

(By vision, I mean having some organ on your body that receives photons and has some signaling pathway that converts photons into a biochemical set of signals)

I think I read in one of my textbooks that vision likely arose from photoautotrophs developing a photosensitive spot in their cells. This was advantageous as the organisms could move into areas where light conditions were favourable
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Rory M♦Jan 23 '12 at 22:07

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What do you mean by vision? How complicated of a system? Photoreception has a long evolutionary/phylogenetic history and can render a fitness advantage without an eye or central processing unit. But as for eyes/brain, that's a different question, I think.
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kmm♦Jan 24 '12 at 0:51

2 Answers
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I'll address the question in the title "At which time did sight evolve for the first time?" by assuming that by the evolution of vision, we mean the evolution of the eye.

Molluscs are an excellent phylum to investigate this question because they exhibit a wide range of eye designs and levels of complexity.

At the most basic level, limpets such as Patella exhibit small patches of photoreceptor cells lying in a relatively flat configuration. Slightly more advanced is Pleurotomaria which has photoreceptors and pigmentation cells held in an eyecup. We then have the pinhole camera style eye as seen in Nautilus (see this post), and more complex eyes with a cornea, retina, and lense such as those seen in squid (e.g. Loligo).

Assuming that "a patch of photoreceptors" in an animal counts as an eye, then we should probably looking for marine invertebrates. The problem here is that if they had only soft body parts we might struggle to identify the oldest examples in sediments.

One candidate for the "earliest eye" might be urbilatarians - the hypothesized last common ancestor of the clade bilatarians - which probably evolved at the end of the Ediacaran period (~555 Myr). An example would be Kimberella (described here) which might or might not have been a mollusc and might or might not have had photoreceptors!

"To me, it also makes sense that the evolution of sight would have accompanied the evolution of advanced brain functions in almost every case." Not necessarily! For instance, think of phototropism: the plant detects the presence of light and uses it to grow towards the light, but that's very simple process regulated by auxins. Or the light-sensitivity (phototaxis) shown by Euglena, a single-celled protist, which reorients its entire body towards a light source. Really simple vision (is there a light, and if so, where is it) can help organisms absorb more energy than rivals, giving them a evolutionary advantage.